PRINCIPLES UNDERLYING METAMORPHIC PROCESSES 615 
opment of a gas phase to proceed to an extent which would be 
inappreciable in the absence of stress. This renders it evident, as 
has been noted on a previous page, that great care must be exer- 
cised in choosing points for purposes of a geologic thermometer; 
for the only points really adapted to this purpose are transitions 
solid-solid (which, moreover, must not be much affected by uniform 
pressure), while systems in which a vapor phase intervenes are 
altogether useless for this purpose. 
METAMORPHISM ACCOMPANIED BY ADDITION OF MATERIAL 
Hitherto we have considered only those cases of metamorphism 
where there was no addition of material during the process; this 
limitation permits us to consider the changes produced in a given 
chemical system by a change in its physical conditions and environ- 
ment. Asa matter of fact, chemical investigation of metamorphic 
rocks' has shown that in many cases the gross composition of the 
original eruptive or sedimentary rocks has remained sensibly 
unaltered by the metamorphic processes;? in other cases, on the 
other hand, it has shown that an addition of material did occur 
during the process of metamorphism. 
Now the addition of material, whether temporary or more or 
less continuous during the process, is often the main cause of 
the changed physical conditions, therefore the cause of the meta- 
morphic process. When this is so the chances for a complete 
transformation of the rock are in general more favorable than when 
temperature and pressure alone are variables. Thus it is that in 
rocks which have been metamorphosed with, or by, the addition of 
foreign material there are peculiarities in mineral content and 
often in texture and structure. In all such cases it is to be noted 
that the gross composition of the rock mow can give us no certain 
idea of the composition of the system during metamorphism, and 
therefore that two rocks which now have the same bulk composi- 
tion may then have represented quite different chemical types. 
™Grubenmann, Die kristallinen Schiefer, 11. Teil, 2. Aufl., 1910. 
2 The occurrence of small changes of composition, such as have been noted, e.g., 
by J. D. Trueman (Jour. Geol., XX, 228 f.), is an occasional, but certainly not a 
general, phenomenon. Such small changes exert no marked specific influence on the 
general result, and consequently are not specially discussed here. 
